A gradient coil is used in magnetic resonance imaging (MRI). A current of the gradient coil has to reach accuracy of tens of ppm over several hundred amperes for imaging requirement. For example, as shown in current profile 100 of FIG. 1, the current Icoil of the gradient coil needs to change from −180 A to +180 A in a short duration (ms range).
In order to accurately control the current of the gradient coil, a gradient amplifier is required to provide an accurate voltage to the gradient coil, wherein the gradient amplifier needs to provide a high voltage during a current transient state in which the current Icoil of the gradient coil changes from one level to another level, and to provide a low voltage during a current steady state in which the current Icoil of the gradient coil remains at a certain level. For example, as shown on FIG. 1, a state in which the current Icoil of the gradient coil changes from −180 A to 180 A or from 180 A to −180 A is the current transient state, and a state in which the current Icoil of the gradient coil remains at either −180 A or 180 A is the current steady state.
However, the conventional gradient amplifier generally generates high frequency harmonics. Therefore, as shown in FIG. 2A, an electromagnetic interference (EMI) filter 220 is needed between the gradient amplifier 210 and the gradient coil 230 to remove the high frequency harmonics generated by the gradient amplifier 210. As shown in FIG. 2B, the conventional EMI filter 220 usually includes two inductors L1, L2, two damping resistors R1 and three capacitors C1, C2, C3.
Up to now, different topologies have been proposed for the gradient amplifier, among which the gradient amplifier with a cascaded H bridge (CHB) topology is widely used in the industry. The existing gradient amplifier 210 with the CHB topology consists of one conventional two-level inverter or more cascaded conventional two-level inverters. FIG. 3 shows a schematic diagram of the conventional two-level inverter 300. As shown in FIG. 3, the conventional two-level inverter 300 includes a first half bridge leg 310 formed by switches QAH and QAL coupled in series and a second half bridge leg 320 formed by switches QBH and QBL coupled in series, wherein the first half bridge leg 310 and the second half bridge leg 320 are connected in parallel to a direct current power supply 330 with a voltage Vbus. The conventional two-level inverter 300 provides an output voltage VAB that is a difference between a voltage VA outputted by the first half bridge leg 310 at a node A located between the switches QAH and QAL and a voltage VB outputted by the second half bridge leg 320 at a node B located between the switches QBH and QBL.
However, in the conventional gradient amplifier 210 consisting of two-level inverter(s), rich high frequency harmonics are usually generated therein. Therefore, in order to attenuate the rich high frequency harmonics generated by the gradient amplifier 210, large inductance values for the inductors L1, L2 included in the EMI filter 220, large resistance values for the damping resistors R1 included in the EMI filter 220 and large capacitance value for the capacitors C1, C2, C3 included in the EMI filter 220 are required. However, large resistance values for the damping resistors R1 result in poor filter efficiency, and furthermore large inductance values for the inductors L1, L2 and large capacitance value for the capacitors C1, C2, C3 would affect overall system control bandwidth.
Further, referring back to FIG. 2A, the gradient coil 230 needs to be connected to ground 240 for purpose of safety and EMI shielding requirement, and thus there is a large overlap area between the gradient coil 230 and the ground 240 which leads to a large stray capacitance Cstray. Since the gradient amplifier 210 has great change in output voltage during operation, a large common mode current Istray flows through the stray capacitance Cstray. The common mode current Istray will be sensed by a high precision current sensor 250 placed between the gradient coil 230 and the EMI filter 220, and act as a high frequency disturbance which results in instability during operation of the MRI system.
Further, the current of the gradient coil 230 will fluctuate in large amplitude because the gradient amplifier 210 has great change in output voltage, which leads to high current ripple of the gradient coil 230.